U.S. Pat. No. 4,774,574, entitled "Adaptive Block Transform Image Coding Method and Apparatus", by S.J. Daly et al, assigned to the assignee of the present application, and the disclosure of which is incorporated herein, describes a mechanism for transmitting an image (in the form of digitally encoded image-representative signals) over a bandwidth-limited communication channel, by means of an adaptive block image transform signal processing scheme that takes advantage of the reduced sensitivity of human vision to noise in the presence of image detail. As part of the compression process carried out at the transmit end of the system, blocks of transform coefficients produced from a (two-dimensional) spatial frequency block transformation process are quantized in accordance with a model of the visibility of quantization error in the presence of image detail. Because the presence of sharp edges between uniform areas will produce energy in the transform coefficients (in approximate proportion to the inverse of the spatial frequency represented by the coefficients), an edge detection mechanism is executed prior to determining the normalization factor. If such an edge is detected, the adaptive component of the normalization process for the block is effectively disabled.
In the preferred embodiment described in the patent, detection of an edge within an image block is accomplished by summing the absolute values of the first m coefficients and comparing the sum to a prescribed threshold value. Alternatively, a more sophisticated edge detector may be implemented by calculating the ratio of the variance of the low frequencies in the image block to the variance of all the frequencies of the block, with a high ratio indicating the presence of an edge. (This variance ratio calculation may be carried out in parallel with the (discrete cosine transform) compression process, and the results employed in the course of determining the normalization factors.)
Further investigation by the present inventor has revealed that a considerable improvement in image quality can be obtained by modifying the more sophisticated edge detector (which makes use of image frequency variances) to include information derived from a histogram of the image block. When the variance information is processed in accordance with the histogram, whether or not an edge is present can be determined with considerably greater precision than when using the simple summation method or the more computationally intensive variance-ratio scheme.